1. Related Applications
This application relates to our co-pending U.S. application Ser. No. 08/032,575 filed Mar. 17, 1990 for an INTERFACE UNIT CIRCUIT WITH ON-CHIP TEST SIMULATION.
2. Field of the Invention
The present invention relates to interface units employed in networks and/or for interconnecting utilization function stations to a bus or cable. More particular, the present invention interface unit defines a protocol for efficiently receiving and transmitting data at stations that are interconnected by a daisy chain bus for operation in a modified Time Division Multiple Access (TDMA) mode of operation.
3. Description of the Prior Art
Heretofore local area networks (LANs) employing a shared bus master ring and interface units were known. In the prior art system the LAN stations contended for use of the bus, thus, units which did not have access to the bus required that the interface unit be provided with very large buffer capacity to store data. The need for buffer capacity increased further when one station was the bus master in all other stations did not have access to the bus. The situation became even more critical when one station had a long string of data to be transmitted or had a priority that denied other stations equal access to the bus in the network.
Such LANinterface units employed a protocol including a plurality of auxiliary bits in the message such as sync or header bits which were used to identify the start of the message and in some cases the message link. This was followed by source and/or destination bits which identified the address of the unit for which the data is intended. The source bits were followed by data which was optionally followed by a message trailer or end of message bits.
Heretofore, Time Division Multiple Access (TDMA) mode of operation networks have been employed to solve the aforementioned need for a large buffer capacity. In some prior art TDMA systems predetermined time slots were generated for each user station on the network. This type of network greatly reduced the utilization factor of the bus when some of the stations on the network left their time slots unused. Another problem with TDMA's network systems is that every time a new station is added to the network system, the protocol must be changed to accommodate new stations and provide new time slots. Heretofore, it was common practice to provide a protocol with more TDMA time slots than stations to provide for future station expansion on the network at the expense of the utilization factor.
Heretofore it was common practice to employ flexible ribbon cables, flexible fiber optic cables, flexible coaxial cables and custom motherboards (backplanes) to provide host to host bus interconnections. LAN network systems were originally designed to overcome the problem presented with the expense of designing custom motherboard bus systems for networks. All of these prior art interconnection systems present mechanical interconnection problems which are substantially avoided by the present novel interface unit.